Conventionally, a lithium ion secondary battery includes an electrode body consisting of positive electrodes and negative electrodes laminated one on the other with separators held between both electrodes. Each separator is generally a porous film made of a resin material such as polyethylene (PE) and polypropylene (PP). This is basically to prevent short circuit from occurring between the positive and negative electrodes and allow ion permeation. Further, this is to cause so-called shut-down in case a battery temperature excessively rises. The separators used herein may be a separator formed with a heat-resistant layer on one surface in order to prevent short circuit from occurring between both electrodes even at the time of shut-down.
One example of such heat-resistant layer is disclosed in Patent Document 1. This heat-resistant layer is made of plate-like boehmite and an organic binder (e.g., claim 2 of Patent Document 1). According to [0033] of Patent Document 1, it is assumed that it is advantageous to mix the plate-like boehmite in the heat-resistant layer in order to prevent short circuit. The reason thereof is explained as that plate-shaped microparticles are likely to be oriented in a fixed plane direction in a heat-resistant layer, which causes a through hole opening at both surfaces of the heat-resistant layer to take a curved or twisted, not straight, shape.